Add important notice about rerouting paths containing regex characters

[hughbris]

Found this important info snippet in an issue comment explaining why some legacy redirects didn't work. IMO it's a detail that needs to be documented in a more obvious place.

[mahagr]

It may be also useful to add some examples on how to match the exact route and how to properly match routes to folders.

[hughbris]

I agree and thinking about this overnight, this was already too much detail for a warning box really. It probably needs a proper subsection to cover properly. I will ponder that.